Corvette Transmission Swap - Overdrive Upgrade

Back in the '80s, when the speed limit was 55 mph, not having an overdriven final gear didn't seem like such a big deal. But times have changed, and anyone who has driven his or her Corvette on the highway for any extended period of time quickly realizes what an inconvenience not having an overdrive gear can be. Enter the Keisler five-speed overdrive transmission available for fitment in your '63 to '82 Corvette using the original shifter and mounting points. It's so sneaky, even your fanatical Corvette resto freaks won't know it's there until you shift into Fifth. Plus, the Keisler five-speed comes in two flavors to suit all needs: the TKO-1 for applications up to 475 hp and the TKO-2 for engines cranking out as much as 650 hp.

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For those of you who aren't sure what "overdrive" means, the term refers to the transmission-output rotation speed versus engine-output speed. In a nutshell, with an overdrive, the transmission-output rpm will be higher than the engine rpm, thus sending more driveshaft output shaft revolutions to the rear wheels with fewer engine rpm. As a result, your Corvette will cruise at higher speeds with less wear and tear on the driveline and your ears. In addition, with fewer engine rpm, less fuel is used. Just think-you'll actually be able to cruise in your Corvette and carry on a conversation without straining your vocal chords or your engine and save gas! Times are good.

Keisler's speed table on page 70 shows the huge impact its overdrive will provide. For example, a 3.73:1 differential ratio combined with the 0.68 overdrive (OD) ratio of the TKO-1 will yield a final ratio of just 2.54 (3.73 x 0.68 = 2.54). This yields a whopping 25-mph speed boost from 52 to 77 mph at 2,500 rpm! A 4.10:1-geared car will jump from 47 to 70 mph on the highway at 2,500 rpm. Keisler states that V-8 musclecars make so much torque they can easily use Fifth gear from 45 mph. The super-duty TKO-2 has a smaller final overdriven ratio of 0.83, which is sometimes beneficial for competition.

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Just having an overdriven gear in a musclecar is a major technological leap forward. But the benefits don't stop there. Tremec designed the TKO with a tire-scorching 3.27 First gear. But if spine-pressing launches aren't your style, you'll appreciate the lower First gear's ability to motivate the car from a stop by letting out the clutch with only the slightest throttle effort. Gone are the days of burning up the clutch or fancy foot moves just to get the car parked. All of this is clearly laid out in Keisler's Speed Table. As you can see, a 3.42:1 differential will have the equivalent launch of a 4.44:1 ratio when combined with the TKO's First gear. A 3.73:1 ratio will instantly become a 4.84:1 launch! This deeper gearing gets the car moving. The remaining gears are more closely spaced at 1.98, 1.34, 1.0, and 0.68 (TKO-1) or 0.83 (TKO-2).

It should be made clear that the Keisler-modified Tremec TKO is vastly different from the standard unit as shipped from Tremec. In addition to the case modification, a complete new patented shifter mechanism is added which brings the shifter to the side, placing the original shifter handle in exactly the correct position to the side and back. The internals are the same as factory delivered, with some additional enhancements for reliability and quiet operation. Spare parts should be readily available for many years to come, as these transmissions are the largest selling aftermarket transmission on the planet. An added benefit of this transmission conversion is that Keisler installs the kit at its Knoxville, Tennessee, facility-just 10 minutes from the Tremec factory.

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So now that you realize Keisler's Tremec TKO five-speed has all of the credentials worthy of your Corvette, let's see what it takes to install one into your favorite Stingray or shark, '63 through '82. The installation shown is for a '66 roadster. Differences in the installation for the mid-year and shark variants are noted. Each Keisler kit is shipped with instructions and each set of instructions finishes with a quality checklist.

Publisher Jerry Pitt, on a recent trip to Keisler's Knoxville headquarters, had a chance to drive a mid-year overdrive conversion. He said, "Nothing was sacrificed in appearance, and the Corvette drove like a modern, manual-shift sports car. In traffic, the Corvette easily ran in the fast lane."

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Corvette Transmission Swap - Overdrive Upgrade

Disconnect the negative battery cable first. Then remove the left and right console side panels and the console top plate. Be sure to note the placement of the window switch connectors (if equipped). Put the transmission in Neutral, then remove the shifter knob and boot, ignition shielding, tach drive, and throttle linkage.

After jacking up the car, remove the driveshaft and shifter. The five-speed comes with a new driveshaft and uses a rail-type shifter instead. Proceed carefully and remove your transmission following factory guidelines.

Keisler strongly suggests that you have the flywheel inspected and resurfaced, then dynamically balanced unless the engine was externally balanced with the flywheel installed. Also, check the release bearing, clutch fork, and ball stud for wear. You'll need to remove the pilot bushing from the crankshaft because the kit comes with a new needle roller bearing.

Check the bellhousing face for parallelism to the block-mating surface. Faces should be within 0.002-inch parallel. If not, have the bellhousing surfaced before proceeding.

Next, install the bellhousing and check for alignment to the crankshaft using a dial indicator. You'll need to send your run-out data to Keisler or you'll void your warranty. Along with the literature for the kit there is a section for Inspection and Correction to help you get the right measurements.

Install the new pilot-bearing assembly by carefully tapping in place. Note: The side of the roller bearing with the rubber seal faces the transmission.

Using a transmission jack or equally stable support, lift the transmission into place with the bellhousing attached by one loosely fastened bolt. Install the release bearing and fork, pressure plate, and clutch disc into the bellhousing.

With an alignment tool, attach the clutch disc and pressure plate to install them onto the flywheel. Tighten the bolts to 35 lb-ft and remove the alignment tool.

Remove the one bolt holding the bellhousing to the trans and bolt the bellhousing to the engine, but don't tighten the bolts yet. Next, install the transmission to the bellhousing. Once everything is in place, torque the bellhousing bolts to 50-60 lb-ft and the transmission bolts to 35 lb-ft. At this point, we filled the transmission with 3 quarts of GM synchromesh transmission fluid (PN 12345349).

There are two transmission mounts used depending on the year of your Corvette. Mid-years...

...and sharks are shown here.

This photo shows the different shifters for mid-years and sharks, depending on the application.

To make the shifter in the right position, Keisler designed this compact shifter so there are absolutely no modifications to the car's floorboards.

You will need to run the new speedometer cable and splice in the reverse-light harness (or you can purchase a complete extension harness from Keisler).

On the topside, install the new shifter handle and check for proper installation. Once everything is installed, no one will be able to tell there's another gear hiding under that slick stick.